Arrange the following in decreasing order of the \(pK_b\) value:
(A) \(C_2H_5NH_2\)
(B) \(C_6H_5NHCH_3\)
(C) \((C_2H_5)_2NH\)
(D) \(C_6H_5NH_2\)
(E) \((CH_3)_3N\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(C_6H_5NH_2 > C_6H_5NHCH_3 > (CH_3)_3N > C_2H_5NH_2 > (C_2H_5)_2NH\)
The smaller the pKb, the stronger the base, so the decreasing order of pKb runs from the weakest base to the strongest base.
(D) \(C_6H_5NH_2\) — first (highest pKb). In aniline the lone pair on N is delocalised into the benzene ring, so it is the least available for a proton. Aniline is the weakest base of the five.
(B) \(C_6H_5NHCH_3\) — second. It is still an aryl amine with the lone pair delocalised, but the CH3 group pushes some electron density (+I effect) on N, so it is a little more basic than aniline.
(E) \((CH_3)_3N\) — third. Alkyl amines are much stronger bases than aryl amines because the +I effect of the alkyl groups raises the electron density on N. In water, however, the tertiary ammonium ion has only one N–H to form hydrogen bonds with water, so it is the least stabilised by solvation, and the three methyl groups crowd the nitrogen. So it is the weakest of the three alkyl amines here.
(A) \(C_2H_5NH_2\) — fourth. The primary ammonium ion is well solvated (three N–H), but there is only one ethyl group to give the +I effect.
(C) \((C_2H_5)_2NH\) — fifth (lowest pKb). Two ethyl groups give a strong +I effect, and the secondary ammonium ion is still solvated through two N–H bonds. This balance makes it the strongest base of the five.
NCERT (2026-27 edition):
Chemical Reactions – “… Larger the value of Kb or smaller the value of pKb, stronger is the base. The pKb values of few amines are given in Table 9.3. pKb value of ammonia is 4.75. Aliphatic amines are stronger bases than ammonia due to +I effect of alkyl groups leading to high electron density on the nitrogen atom. Their pKb values lie in the range of 3 to 4.22. On the other hand, aromatic amines are weaker bases than ammonia due to the electron withdrawing nature of the aryl group.”
